That Hovering Fish May Want a Massage

While we often admonish divers and divemasters who manhandle fish and other marine creatures, we were intrigued by a study in Nature Communications that says fish get just as stressed as humans do, and therefore may be in equally need of a massage.

Surgeonfish make regular use of cleaner wrasse to remove their parasites and dead skin. Marta Soares of the ISPA University Institute in Lisbon, Portugal, noticed that the cleaners seem to offer another service too: They can placate an agitated surgeonfish by rubbing back and forth on its pelvic and pectoral fins.

Soares and her team wanted to see if it was the social interaction or the feeling of the massage that kept the surgeonfish at ease. To test this, they studied two groups of eight surgeonfish. They confined each fish in a small bucket for a short period to simulate stresses they encounter in the wild. They then placed the surgeonfish into tanks with a model cleaner fish. One group was given a stationary model, the other a model that moved back and forth, and so could provide physical stimulation All surgeonfish readily approached the model, but those in the tank with the moving model were able to position themselves beneath it and use its fake fins to gain a back rub. These fish were more relaxed, with lower measurements of the hormone cortisol, which is released in response to stress.

Soares says that the tactile stimulation by cleaner wrasse definitely offers the client surgeonfish a benefit, and that her research may mean that pathways for sensory information processing in fish are more similar to humans that previously thought. "Humans go to have massages when we feel sick or just to feel better, so maybe the reasons are basically the same," she says.

Her results come as no surprise to diver Dan Lufkin (Frederick, MD) who writes that several species seem to find divers interesting, and will often hang around to be scratched and patted. "Groupers, in particular, like to have their bellies rubbed. Recently, diving off Little Cayman, I met a grouper about 30 feet down, which swam below me for 20 minutes. I could put my hands around it and squeeze. The fish would shimmy out ahead, then come back into position for another treatment. Personally, I could no more eat a grouper than I could a cocker spaniel."